1. Field of the Invention
The present invention relates to an image forming apparatus in which toner images formed by an image former are transferred onto an intermediate transfer medium which moves in a predetermined direction and the toner images are temporarily carried.
2. Description of the Related Art
The apparatus described in JP-A-2004-109617 for example is known as an image forming apparatus of this type. In this image forming apparatus an image forming station, in which a charger, an image writer and a developer are disposed around a latent image carrier such as a photosensitive drum, is disposed for each color along a transfer medium such as a transfer belt. Toner images formed by image forming stations are superimposed one atop the other on the transfer medium, whereby a color image is formed.
By the way, one of serious problems arising in an image forming apparatus comprising plural image forming stations is a color misregistration. This occurs as the transfer positions to which toner images formed by different image forming stations are transferred get displaced from each other, and shows as a change of hues. To solve this problem, reference pattern images (hereinafter referred to as “registration marks”) for detecting a color misregistration may be formed in advance on a transfer medium and registration marks may be detected with an optical sensor to thereby calculate position information regarding the registration marks and align the respective toner images based on this position information (color misregistration correction). To be more specific, the position information regarding the registration marks is acquired through detection of edge portions of the registration marks. In short, while the transfer medium carrying the registration marks moves, (1) the position information is detected when upstream edge portions of the registration marks move passed the optical sensor and the level of an output from the optical sensor exceeds a pre-selected threshold level, or (2) the position information is detected when downstream edge portions of the registration marks move passed the optical sensor and the level of an output from the optical sensor exceeds a pre-selected threshold level.
Further, an image forming apparatus which uses an intermediate transfer belt may be a color image forming apparatus of the tandem type for instance. In this color image forming apparatus, image forming stations for yellow, magenta, cyan and black are disposed along the intermediate transfer belt, and toner images formed by the image forming stations are superimposed one atop the other on the intermediate transfer belt, whereby a color image is formed. The color image is then transferred at proper timing onto the recording medium.
The intermediate transfer belt is thus a major component of the image forming apparatus, and choice of a belt base material is important to secure a mechanical strength, a mechanical accuracy and the like in particular. Noting this, a conventional approach is to use a PC (polycarbonate) resin mainly made of polycarbonate considering a mechanical strength, a mechanical accuracy, etc., as described in JP-A-4-313757 for example.